Test environment stability improvements (#703)

* Add climate_control gem and test helper

* Replace ENV mods in upgrades test

* Replace ENV mods in registrations test

* Remove ENV references in hostings controller

* Update ENV refs in mailer test

* ActiveStorage cleanup

* Consolidate queue config so appropriate adapter runs in test environment

* Make test environment more explicit

* Centralize self hosting config

* Remove flaky system test

Gemfile

@@ -51,12 +51,12 @@ group :development, :test do
   gem "debug", platforms: %i[ mri windows ]
   gem "brakeman", require: false
   gem "rubocop-rails-omakase", require: false
-  gem "dotenv-rails"
   gem "i18n-tasks"
   gem "erb_lint"
 end
 
 group :development do
+  gem "dotenv-rails"
   gem "hotwire-livereload"
   gem "letter_opener"
   gem "ruby-lsp-rails"
@@ -69,4 +69,5 @@ group :test do
   gem "mocha"
   gem "vcr"
   gem "webmock"
+  gem "climate_control"
 end